The mosquito responsible for the West Nile virus flourished during the summer's record heat and drought. Now, officials are concerned about emerging signs that a widespread outbreak may be on the horizon in Illinois.

Updated figures from the state Department of Public Health show extremely high numbers of the Culex pipiens species have tested positive for the disease — 71 percent in DuPage County and nearly 60 percent in Cook, the health department reported.

Although the 27 cases of West Nile virus in Illinois don't represent a particularly high number, experts start to get anxious when just 10 percent of samples of virus-carrying mosquitoes test positive.

The reason, said Linn Haramis, program manager of vector control for the health department, is that history suggests that the 10 percent infection rate is a strong indicator the percentage is going to accelerate rapidly over the summer.

The rate of Culex pipiens mosquitoes statewide that had the West Nile virus stood at 25 percent Tuesday, Haramis said. Last year, that percentage was 8 percent, he added.

"To use the police analogy," Haramis said, "if you've got more criminals walking around the street, you've got more potential for street crime."

Illinois' troubling signs emerge as the Centers for Disease Control and Prevention on Wednesday released national figures showing 1,118 cases and 41 deaths have been reported to the CDC — the highest number of cases through the third week of August since the disease was first detected in this country in 1999.

The CDC figures represent a substantial jump in numbers from last week's tally of 693 cases and 26 deaths. And the number of reported cases through the third week of August this year is nearly three times higher than the average over the last 10 years, according to the CDC.

The 27 cases reported in Illinois, which include one death, are concentrated in the Chicago area and are more than double the total number of cases — 11 — reported Aug. 15. The state recorded 34 cases of the virus last year.

With August considered a high-risk time for West Nile outbreaks in the Midwest, Illinois health officials repeated their warnings about the tawny insect with the distinctive, high-pitched buzzing sound.

"People need to listen to us," Haramis said. "There is a high level of activity now, and this is a disease that could have a significant impact on your life. It could leave you in a wheelchair."

The CDC's Dr. Lyle Petersen gave a more dire assessment.

"The number of West Nile virus disease cases in people has risen dramatically in recent weeks and indicate that we're in the midst of one of the largest West Nile virus outbreaks ever seen" in the U.S., Petersen said Wednesday.

Five states are responsible for about 75 percent of the West Nile cases — Texas, Mississippi, Louisiana, South Dakota and Oklahoma, the CDC said. Almost half of the cases were reported in Texas, where the drought has been unusually severe. Only Alaska, Hawaii and Vermont have reported none this year.

Southern states appear to be affected earlier than Northern states, possibly due to an earlier spring season, Petersen said.

But officials said they do not know why certain Southern cities such as Dallas, which has started aerial pesticide spraying, have been hit so hard, other than that they appear to have the right combination of birds, mosquitoes and susceptible people.

Mosquito activity is highly weather-sensitive. Cooler temperatures and heavy rain reduce the number of Culex pipiens, experts said. Downpours can wash away larvae growing in places such as catch basins and gutters. That didn't happen this summer.

But high temperatures allowed the virus to replicate quicker, building to dangerous levels inside the mosquito, which infect people through its saliva, experts said.

Even the warmer winter may have helped. The mild weather then and in the early spring, combined with the hot summer, might have fostered conditions favorable to spread the virus, according to CDC officials.

"It's a banner year for West Nile," said Richard Pollack, a public health entomologist with the Harvard School of Public Health. "Not such a good year for people."

Cases usually flare in the summer because the illness is most often transmitted from infected birds to people by mosquitoes.

But the CDC notes that about 80 percent of those infectedare asymptomatic.

The virus is most likely to harm those older than 50 and who have medical conditions such as cancer, diabetes, hypertension and kidney disease. Organ transplants also are particularly vulnerable.

Symptoms include fever, headache, body aches, joint pains, vomiting, diarrhea or rash. Less than 1 percent will develop a serious neurological illness such as encephalitis or meningitis.

Once infected, a person is assumed to have lifelong immunity, which may wane in later years, CDC officials said. There is no specific treatment.

The best strategy to defeat the mosquito, health officials say, is to wear long sleeves and pants and use mosquito repellent when outdoors in the evening. It's also recommended to clear all places where standing water can accumulate.

Although impossible to predict when the West Nile cases will peak this year, CDC officials said infections typically crest around mid-August. Reporting lags infection, which often means those infected now may not be recorded for weeks.

"We expect many more cases to occur and the risk of West Nile virus infection will probably continue through the end of September," Petersen said.

For the time being, mosquito control businesses have seen a spike in activity, said Andy Fuller, operations manager for Skeeter Beaters, based in Chicago.

Now that rain has returned to normal levels in recent weeks, the more familiar swarms of Aedes or floodwater mosquitoes have surfaced. They don't carry West Nile virus but often are considered more annoying because they move in swarms and sting incessantly.

Fuller said the number of service calls his business has received are double what they were in early July.

"We're getting kind of a perfect storm for" Culex and Aedes species, Fuller said. "All the different mosquitoes are in full swing now … and it's scary for some people."